The use of an aqueous chelated iron catalyst solution for removing hydrogen sulfide from a gas stream is well known in the art. The solution is contacted with the hydrogen sulfide-containing gas to effect oxidation of the hydrogen sulfide to elemental sulfur and concomitant reduction of the iron from its ferric state to its ferrous state. The solution is regenerated for reuse by contacting it with an oxygen-containing gas to oxidize the iron from its ferrous state to its ferric state. The following U.S. patents are illustrative of a process of this type:
______________________________________ Inventor U.S. Pat. No. Date ______________________________________ Hartley et al 3,068,065 Dec. 11, 1962 Pitts et al 3,097,925 July 16, 1963 Meuly et al 3,226,320 Dec. 28, 1965 Roberts et al 3,622,273 Nov. 23, 1971 Roberts et al 3,676,356 July 11, 1972 Sibeud et al 3,897,219 July 29, 1975 Salemme 3,933,993 Jan. 20, 1976 Meuly 4,009,251 Feb. 22, 1977 Mancini et al 4,011,304 Mar. 8, 1977 Sibeud et al 4,036,942 July 19, 1977 Hardison 4,076,621 Feb. 28, 1978 Winkler 4,091,073 May 23, 1978 Thompson 4,189,462 Feb. 19, 1980 Thompson 4,218,342 Aug. 19, 1980 Hardison 4,238,462 Dec. 9, 1980 Lynn et al 4,278,646 July 14, 1981 Lynn 4,330,478 May 18, 1982 Blytas et al 4,348,368 Sept. 7, 1982 Diaz 4,368,178 Jan. 11, 1983 Primack 4,374,104 Feb. 15, 1983 Diaz 4,382,918 May 10, 1983 Diaz 4,388,293 June 14, 1983 Blytas 4,390,516 June 28, 1983 Diaz 4,400,368 Aug. 23, 1983 Diaz 4,402,930 Sept. 6, 1983 ______________________________________
The above-listed Roberts et al patents refer to Czechoslovakian Pat. Nos. 117,273, 117,274, and 117,277 as also disclosing the use of aqueous chelated iron solutions for the same purpose. In addition, British Pat. Nos. 855,421, 99,799, and 99,800 also relate to hydrogen sulfide removal processes utilizing aqueous chelated iron solutions.
Many of the prior art patents in this field appear to be based on a limited amount of small-scale experimental work. Consequently, the disclosures of such patents do not recognize or purport to solve the principal operating problem that has been encountered in prolonged large-scale operation of a hydrogen sulfide removal process of this type utilizing an aminopolycarboxylic acid chelating agent. For example, many of the above-mentioned prior art patents express concern about the formation of acidic oxides of sulfur, such as thiosulfates, particularly at the neutral or alkaline pH levels most effective for hydrogen sulfide removal. The prior art has proposed the addition of organic amines, buffering agents, and other additives to the chelated iron solution to inhibit or minimize thiosulfate formation. Other expedients have also been suggested, such as periodic withdrawal of part of the recirculating chelated iron solution to control the accumulation of thiosulfate, or operating the process at a lower pH level to discourage thiosulfate formation.
In commercial-scale operation of a process of this type, however, we have found that thiosulfate formation and accumulation is not a troublesome problem. As explained in more detail below, we have found that chemical degradation and loss of the aminopolycarboxylic acid chelating agent necessitating addition of replacement chelating agent is the most significant operating problem affecting the economic feasibility of prolonged large-scale operation of the process.
A few of the prior art workers have acknowledged that chelated iron solutions are "unstable" and that undesirable precipitation of iron compounds may occur. For example, British Pat. No. 999,799 recommends close adjustment of pH to avoid breakdown of the chelate complex. British Pat. No. 999,800 suggests careful control of the regeneration of the catalyst solution to avoid over-oxidation of the iron chelate. U.S. Pat. No. 4,189,462 to Thompson indicates that restricting the molar ratio of EDTA to iron is an important consideration in avoiding breakdown of the chelate molecule. U.S. Pat. No. 4,330,478 to Lynn suggests the use of specific types of aliphatic polycarboxylic acid chelating agents that are allegedly more resistant to oxidation. U.S. Pat. No. 3,622,273 to Roberts et al discloses the addition of selected buffering agents to maintain the pH at a relatively high level at which the chelated iron complex is said to be more stable. U.S. Pat. No. 4,278,646 to Lynn et al suggests the addition of selected amine salt stabilizers to achieve chelate stability at low pH levels. The Diaz U.S. Pat. Nos. 4,382,918, 4,388,293, and 4,400,368 propose the addition of various sulfur-containing and nitrogen-containing compounds as stabilizers to reduce the rate of chelate degradation, but the reported data show only a relatively modest improvement in the chelate loss.
Before the present invention, the prior art has not provided an effective, environmentally acceptable, and inexpensive solution to the problem of chelate degradation. Moreover, there has been no adequate explanation of the mechanism of chelate instability in a hydrogen sulfide removal process.